ESCI 101: LectureESCI 101: LectureThe Rock Cycle The Rock Cycle & Igneous Rocks& Igneous Rocks

February 23, 2007February 23, 2007

Copy of this lecture will be found at:http://www.owlnet.rice.edu/~esci101

With Some Graphics from Press et al.,

Understanding Earth, 4th Ed. (Copyright © 2004 by W. H. Freeman & Company)

Enchanted Rock

The Rock The Rock CycleCycle

-Melting & Intrusion-Solidification of melt-Mountain Building-Uplift & Exposure-Weathering-Erosion & Transport-Deposition & Burial-Metamorphism-Melting & Intrusion

Fig 4.9

ESCI 101 - “Pop” Homework #4(Due Wednesday, 2/28/07, 5 PM)

• Assignment:- For this exercise, trace the

alternate pathway of the rock cycle: (1) igneous rocks are formed, (2) become metamorphosed, (3) exposed, and (4) evolve into sandstone. Describe the main processes and events that might take place at each step along the way, and the plate tectonic settings in which they are likely to occur. (Your answer should not exceed one page.)

- See the accompanying plot on the following page.

Pop-HW#4Pop-HW#4 An An

Alternate Alternate Rock Rock CycleCycle

-Melting & Intrusion-Solidification of melt-???-???-???-???-???--> Sandstone

Fig 4.9

X

XX

(a) The Rock (a) The Rock CycleCycle

Fig 4.9

E.g., Convergent

Plate Boundary-Subducting

slab-Mantle melting-Bouyant rise

of melt

(b) The Rock (b) The Rock CycleCycle

Fig 4.9

E.g., Convergent Plate Boundary- Solidification

of melt- Volcanic activity

Intrusive rocks

Extrusive

rocks

Genetic Classification of Genetic Classification of Igneous RocksIgneous Rocks

Granite Gabbro

• IntrusiveIntrusive: crystallized from slowly cooling magma intruded within the Earth’s crust; e.g. granite, gabbro.

Rock Rock TexturTextur

eeFig. 5.1

Intrusive: Course-grained,

interlocking

Genetic Classification of Genetic Classification of Igneous RocksIgneous Rocks

Rhyolite Basalt

• ExtrusiveExtrusive: crystallized from rapidly cooling magma extruded on the surface of the Earth as lava, …

Rock Rock TexturTextur

eeFig. 5.1

Intrusive: Course-grained,

interlocking

Extrusive: Fine-grained,

discrete crystals, often

glassy

Genetic Classification of Genetic Classification of Igneous RocksIgneous Rocks• ExtrusiveExtrusive: … or erupted as

pyroclastic material, i.e., fragmented pieces of magma ejected and cooled in the air.

Pumice

Scoria

Ash

Formation of Igneous Formation of Igneous RocksRocks

Pyroclasts

Extrusive

Intrusive

Porphyry: partially crystalline

Fig. 5.3

Process of Process of IntrusionIntrusion• Intrusive rocksIntrusive rocks fracture and cross-

cut the “country rock”. • Igneous rocks cool quickly near the intrusive contacts, and cause contact metamorphism.

Fig. 5.2

Composition and Composition and Classification of Classification of Igneous RocksIgneous Rocks

• Chemistry: e.g. % SiO2

• Mineralogy: e.g.–Felsic - high silica–Intermediate - intermediate silica

–Mafic - low silica–Ultramafic - very low silica

(continental crust)

(oceanic crust)

(mantle)

Felsic Igneous Rocks:Felsic Igneous Rocks:

Igneous rocks rich in mineralsIgneous rocks rich in minerals highhigh in silicain silica andand low inlow in iron and magnesium. They iron and magnesium. They

include:include:

GraniteGranite RhyoliteRhyolite

Mafic Igneous Rocks:Mafic Igneous Rocks:

Igneous rocks rich in mineralsIgneous rocks rich in minerals lowlow in silicain silica andand high inhigh in iron and magnesium. They iron and magnesium. They

include:include:GabbroGabbro BasaltBasalt

Common Common MineralsMinerals

Intermediate Igneous Intermediate Igneous Rocks:Rocks:

Igneous rocks Igneous rocks intermediateintermediate in in composition between felsic and composition between felsic and

mafic igneous rocks. They mafic igneous rocks. They include:include:

Granodiorite (Dacite)Granodiorite (Dacite)Diorite (Andesite)Diorite (Andesite)

Ultramafic Igneous Ultramafic Igneous Rocks:Rocks:

Igneous rocks with Igneous rocks with very lowvery low silica content, consisting silica content, consisting

dominantly of mafic minerals. dominantly of mafic minerals. The most common ultramafic The most common ultramafic

rock is:rock is:Peridotite (no extrusive equivalent)Peridotite (no extrusive equivalent)

Fig. 5.4

CompositioCompositional nal

ClassificaClassificationtion

Granite

Granite

Quartz

Orthoclase

Biotite

Plagioclase

CompositioCompositional nal

ClassificaClassificationtion

Granite

Granite

Granodiorite

Quartz

Amphibole

Plagioclase

CompositioCompositional nal

ClassificaClassificationtion

Granite

Granite

Granodiorite

Diorite

Plagioclase

Amphibole

CompositioCompositional nal

ClassificaClassificationtion

Granite

Granite

Granodiorite

Diorite

Gabbro

Plagioclase

Pyroxene

CompositioCompositional nal

ClassificaClassificationtion

Granite

Granite

Granodiorite

Diorite

Gabbro

Peridotite

Pyroxene

Olivine

Table. 5.2Felsic Intermediate Mafic

Granite GranodioriteDiorite Gabbro

BasaltAndesiteDaciteRhyolite

Viscosity

Melting Temperature

How do magmas form?How do magmas form?

When rocks melt (or partially melt).

When do rocks melt?When do rocks melt?

When the temperature exceeds the melting point of the rock or

some minerals within the rock.

Factors that Affect Factors that Affect Melting of Minerals (and Melting of Minerals (and

Rocks) Rocks)

• CompositionComposition: Felsic minerals melt at lower temperatures than mafic minerals

• PressurePressure: Increased pressures raises melting points

• Water ContentWater Content: Increased water content lowers melting points

The Formation The Formation of Magma of Magma ChambersChambers

Partial meltingPartial melting

Less dense magmaLess dense magma

Magma risesMagma rises

Magma pools in Magma pools in magma chambermagma chamber

Some minerals melt before others. Results in mixture of melt and solid.

Melt is less dense than solid. Low density minerals tend to melt first.

Buoyant melt migrates through rock pores and fractures.

- Covers 15,400 mi2 !! - Composition

- Granite- Monzonite- Granodiorite- Diorite- Even Gneiss

- All in one “magma chamber”

Why do magmas have such Why do magmas have such different compositions different compositions - - even when derived from a even when derived from a single “parent magma”single “parent magma”

e.g., granite, granodiorite, diorite

Magma DifferentiationMagma Differentiation

The process by which rocks of various compositions can arise from a uniform

parent magma

Occurs because different minerals crystallize at different temperatures (i.e., the opposite of

partial melting)

Tune in Wednesday for the exciting conclusion!